Protocol Analyzers are used to capture, examine, and debug the complex protocols used by storage and networking busses. Oscilloscopes are used to view waveforms and verify signal integrity. Modern point-to-point busses, such as those operating in accordance with the Serial Attached SCSI (SAS) protocol, transfer data at a rate of gigabytes per second on each of multiple differential transmission lines, and employ complex protocols for equalizing or training the transmitters in order to compensate for the frequency dependent characteristics of the cable or transmission line.
Tapping into or splitting a transmission line degrades transmitted signals because splitting redirects a portion of the signal resulting in power loss, and tapping changes the impedance of the transmission line, creating signal reflections. As an alternative to tapping into a transmission line, protocol analyzers often use a technique of terminating the received signal and then retransmitting it again as a clean new point-to-point signal with full strength. Such a technique will usually change the signal waveform somewhat, so performing this technique multiple times on the same signal in order to attach multiple instruments is undesirable.
One aspect of modern GHz busses which adds to the difficulty of troubleshooting while maintaining high signal integrity is the large number of signals involved. For example, SAS hosts and expanders are typically four lanes wide, with each lane consisting of two differential pairs; one pair in each direction. Thus a typical SAS bus involves sixteen signals all operating at GHz speeds. The large number of signals makes it more difficult to tap or split without introducing stubs or discontinuities into the transmission line.